In the past, buildings would have several cabling systems, respectively for different types of communications systems. For example, telephone wiring was used for voice, coaxial cable for data and video networks, multi-pair cabling for RS232/RS422 control data, etc. With all of the separate costs involved, this became a very inefficient and costly way to install these systems. A solution was to install a standard cable and connector system throughout a building, which could, with some additional equipment, be used to support all, or most of the different types of communication systems in use in the building. This standard cable and connector system is called a “Structured Cabling System” (“SCS”).
The SCS is a set of cabling and connectivity products that integrate voice, data, video and various building management systems (“BMS”), such as safety alarms, security access, energy systems, etc. Characteristics of an SCS include an open architecture, standardized media and layout, standard connection interfaces, adherence to national and international standards, and total system design and installation. Typically, SCS cable is a multi-pair cable made up of unshielded twisted pairs (“UTP”) of insulated copper conductors. A typical SCS cable includes four such twisted pairs. A typical building has a plurality of SCS cables, perhaps dozens or even hundreds, extending throughout the building. Apart from the SCS, the voice, data, video and BMS have nothing in common, except for similar transmission characteristics (analog or digital data signals) and delivery methods (conduit, cable, tray, raceway, etc.) that support and protect the cabling.
The SCS is constructed and installed under a common set of rules. By constructing all SCS systems under these rules, everyone knows how the SCS will perform. These rules are changed from time to time by groups representing the industries involved. The rules are available in order to provide a standard that everyone can work from.
Presently, there are products that allow standard CCTV equipment to interface with a SCS. This equipment generally uses 24 VAC, which is appropriate for large scale systems. There is also a need, however, for the use of 12VDC cameras in certain systems.
Large scale systems almost exclusively use 24VAC because almost all camera systems use a centralized power supply and 24VAC cameras are not as sensitive to the voltage drop problem as DC cameras are. Also, a mix of both DC and 24VAC cameras, would mean putting two separate power supplies in and routing cables separately.
Using the SCS as a method for camera installation is a relatively new method as was explained earlier. All prior systems within the SCS operated on a single voltage and that was commonly 24VAC. In an attempt to provide for both 12 VDC and 24VAC cameras in a single power supply system a rectifier and regulator were used at the camera end. This system is shown in FIG. 1. This was an unsatisfactory attempt to solve the problem because switching noise and power supply ripple flow into the video signal ground causing interference in the video picture.
FIG. 1 shows a prior art 24VAC camera 10 with an isolated power supply at the camera. Note that it does not provide a path for current flow back to the 24VAC source 12 though the video ground because they are isolated. FIG. 1 also shows that with two 12VDC cameras 14 connected to the system a current path is provided. DC cameras use the same ground for both the power and the video, therefore providing a path back to the transformer through the second camera's video ground connection. The common video ground in the Intermediate Distribution Frame (IDF) closet closes a loop. Current drawn though the filter capacitors and the camera load is coupled into the camera ground connection. Noise from the 24VAC current changes through the cycle and the noise of the diodes switching are felt in the video pair and that noise is imparted on to the video signal and displayed in the video picture.
Attempts to fix the problem have focused on tying the camera ground to earth ground. With a good earth ground at the camera and at the IDF closet a third path for current is created. If the third path to ground is of low enough impedance it will usually fix the problem. But this may not always work and even when it does it requires an additional connection outside of the SCS. Of course, additional connections outside of the SCS are not the standard.
The invention presented here provides a method for the transmission of power and control data to the camera as well as video from the camera, over the SCS. It also allows for the use of 24 VAC and 12 VDC cameras in the same system. The ability to mix and match both 12 VDC and 24 VAC cameras in a single installation provide a completive advantage in both price and performance.